1. Field of the Invention
The present invention relates to producing a low-calorie sweetening substance, more particularly, to crystallization of the .alpha.-L-aspartyl-L-phenylalanine methyl ester.
2. Description of the Related Technology
.alpha.-L-aspartyl-L-phenylalanine methyl ester, hereinafter referred to as aspartame, is a low-calorie sugar substitute and is industrially synthesized in several different processes. The synthesized aspartame in solution is then isolated most commonly by crystallization followed by dewatering and drying processes. Further treatments or processes can be applied to the aspartame to provide various consumer products.
Crystallization occurs by cooling a solution or by evaporating the solvent thereof. In cooling crystallization, as the solution is cooled, the solute, herein aspartame, is supersaturated, which causes formation and growth of crystals. Generally, obtaining coarse and uniform crystals is the goal. In theory, this can be accomplished by cooling the solution with an infinitesimal amount of heat transfer, which would take a very long period of time.
In practice, an aspartame solution is cooled and the crystallization is carried out by heat exchanging the solution with a coolant having a significantly lower temperature than the solution. The temperature of the solution is readily homogenized by stirring or agitating the solution, in which the time for crystallization is reduced. However, the cooling with a relatively large temperature difference does not produce coarse and uniform crystals.
Further, U.S. Pat. No. 5,543,554 discusses that forced flow or agitation of the aspartame solution during the cooling crystallization deteriorates crystal properties in removing liquid therefrom. Based on this, the patent suggests that no forced flow or agitation has to be applied during the crystallization of the aspartame. However, the cooling is commonly carried out by heat-exchange between the aspartame solution and a coolant via a surface. Without forced flow or agitation, local crystallization occurs near the heat-exchange surface before the temperature of the solution is homogenized. The locally formed crystals are likely to precipitate on the surface and form scales. The scales in turn deteriorate the cooling efficiency of the solution by acting as an insulating layer, as well as require laborious work to remove afterward.